THE NATURE OF PHOTOSYNTHESIS 115 



velopment of color in marine plants i^rown under light of different color, 

 Richter emphasizes that only the photosynthetic activity of the variously 

 colored plants under the different conditions of illumination is of real 

 biological significance. Richter therefore takes the stand that the distribu- 

 tion of the different colored algae is a matter of light intensity, quite 

 analogous to "light" and "shade" land plants. The vertical zonal dis- 

 tribution is a matter of light intensity or "Lichtgenuss" in the sense of 

 Wiesner, and the other pigments in the plants play no essential role in 

 photosynthesis. Thus Richter concludes that only the chlorophyll proper 

 plays a role in photosynthesis. 



Regarding the phenomenon of zonal distribution of aquatic plants 

 primarily from the viewpoint of photosynthesis, an aspect of much interest 

 is whether the blue and red complementary pigments, phycocyanin and 

 phycoerythrin play a role in the photosynthetic process. It is evident from 

 the work of Richter, just cited, that consideration must be given the in- 

 tensity of the light as well as the wave-length in interpreting the com- 

 plementary color adaptation phenomenon. Also from what has already 

 been said it is evident that attention must be paid to the intensity and 

 wave length of the light not only during the course of the determinations 

 of photosynthetic rates but as well for a period prior to making such de- 

 terminations. Considerable information on these cjuestions has been 

 gained through an investigation by Harder.^^ He used a blue-green alga, 

 Phonnidium foveolarum which in red light becomes green and in blue 

 light takes on a purple color. Boresch ^- has shown that the purple form 

 contains phycoerythrin and that the green form contains the blue pigment 

 phycocyanin. By means of light filters, Harder obtained blue and red 

 Hght the intensity of which he measured with a Rubens thermopile. The 

 rate of photosynthesis was determined from the oxygen content of the 

 water by Winkler's method. 



Harder upholds the conclusions of Englemann of complementary 

 adaptation; the purple varieties of the alga, containing the red pigment, 

 phycoerythrin, have a higher rate of photosynthesis in blue light than in 

 red, while the blue-green varieties containing the blue pigment, phycocyanin, 

 show higher rate of photosynthesis in red light than in blue. These re- 

 sults apply also when equal intensities of the different colored lights are 

 used ; the rate of photosynthesis is relatively and absolutely higher in those 

 wave-lengths which are complementary to the color of the plant. 



That the intensity of light in which plants have been growing, and 

 to which they have in a sense become adjusted, is a very important factor 

 in the rate of photosynthesis has been recognized for some time. This is, 

 of course, the basis of the behavior of "hght" and "shade" plants and 

 has already been touched upon. It is, in fact, possible to produce arti- 

 ficially with the same species, as far as their light requirements for photo- 



" Harder, Zcit. f. Bof., 15, 305 (1923). 



*' Boresch, Arch. f. I'roslistcnkundc, 44, 1 (1921). 



